The present invention relates to a method for calendaring a moving web of paper or board.
In calendering, a moving web of paper or board is treated in a nip formed between revolving rolls. The amount of treatment occurring in the web being calendered is affected by the calendering conditions and the qualities of the web being treated. Variables related to the calendering effect are those suited for the control of the calendering process, such as the temperature and speed of rotation of the rolls, as well as the linear load imposed on the web in a nip between two rolls. In regard to the qualities of the web being treated, such as the web moisture content, temperature, basis weight and density are those affecting the amount of web treatment taking place. For instance, moist and warm paper is treated more effectively than dry and cold paper under the same calendering conditions. Respectively, it is possible to cause a larger density change in a paper approaching with a low density the calender under the same calendering conditions than what is possible in a paper already having a high density.
The properties of a moving web of paper or board entering a calender are reflected, not only in the processability of the web, but also in the calendering process conditions. An example of such a mutual interaction is heat transfer between the web and the calender rolls. The amount and direction of heat transfer are affected by the temperature and moisture of the web being calendered. Herein, it is even possible that an uneven temperature and/or moisture profile of the web entering the calender may gradually change the temperature profile of the rolls. The resulting thermal expansion of the rolls in turn changes the peripheral profile of the rolls, whereby also the loading profile of the nip changes.
In control theory, control strategies can be categorized in two different major classes known either as feedback control or feedforward control depending on how the control signal is applied. In feedback control, the measured value of the process variable to be controlled is compared with a set value and, whenever necessary, the value of a control variable in the control circuit is changed so as to bring the difference between the set value and the measured value to a minimum. In feedforward control, the value of the control variable is changed on the basis of some other input signal value than that of the actual process variable being controlled. Generally, the input signal in feedforward control is some measurable disturbance of the process whose magnitude cannot be directly affected by the control circuit, but whose effect on the actual process variable being controlled can be compensated for by way of proper tuning of the control circuit.
Generally, an effective control system can be configured by combining feedback control with feedforward control. Herein, feedforward control is used in a predictive manner to compensate for the effect of known process disturbances by way of utilizing a priori information on the interdependence between the disturbance and the process variable being controlled and, on the other hand, between the control action and the process variable being controlled. At the same time, feedback control is used to assure that the process variable being controlled stays close to its set value. The latter control circuit is mandatory, because not all disturbance effects are measurable and, moreover, feedforward control is inherently slightly inaccurate.
In a multivariate control system based on the use of two or more control variables, the benefits of feedforward control are accentuated. By utilizing information obtained on the state of disturbance parameters, the chances are improved to select those control variables that offer optimal disturbance compensation. Simply, if the cause of a disturbance is included in the model of the control algorithm, the better are its possibilities of reaching an optimal correction to the situation.
As mentioned above, it is known in the art that the changes in the qualities of a web passing a calender are effected, not only by the calendering conditions, but also by the properties of the web entering the calender. However, it is customary to measure in a paper- or board-making process the qualities of the moving web only after each section, for instance, at the upwinder of a paper-making machine or of an off-line calender. Since the qualities of the web being calendered are not conventionally measured before the calender, variations in these qualities represent unknown disturbances to the calender control systems and their effect can be identified only from web quality measurements performed downstream from the calender. Hence, control strategies applicable to a calender must be implemented using feed back control alone, with the penalty that such a control scheme can react only after the effect of disturbances becomes explicit on the measurement value of the process variable being controlled.
The fact that the qualities of the web being calendered are not conventionally measured upstream in front of the calender also complicates the identification of causes behind problems possibly occurring in the web profile. The contribution of the calender itself in a web profile problem can be identified by way of temporarily eliminating the effect of the calender from the process measurements. In conventional measurement arrangements, this can be accomplished only by performing measurements at an upwinder on such an incoming web that has passed the calender with the calender nips set open. However, this is an unusual test which is launched only after the problem has been ongoing for quite a time so that production losses have already occurred due to the problem.
In reality, a web profile problem rarely is so simple that its origin can be traced to the calender alone or, respectively, in front of the calender alone. Namely, it is also possible that an uneven profile of the web entering the calender may gradually change the temperature profiles, the peripheral roll profiles and the loading profiles of the calender, whereby a simple web profile problem originating upstream from the calender may change the calendering conditions into such a direction that the calender itself begins to cause profile problems.
It is an object of the present invention to provide an entirely novel type of calendering method capable of overcoming the problems hampering the above-described prior-art techniques.
The goal of the invention is achieved by virtue of measuring the qualities of the web to be calendered also upstream from the calender, whereby the measurement results can be utilized in feedback control by way of directly modifying the factors that affect web processability. Such qualities subject to measurement may be, e.g., the moisture content, temperature, basis weight and density of the web. Then, deviations in such quality variables measured upstream from the calender can be considered as a category of disturbances known to the control system. Resultingly, the common feedback control can be complemented with feedforward control scheme capable of handling these detectable disturbances. Inasmuch the control algorithm of a feedforward control system attempts to compensate for the effect of a detected disturbance in a process variable being controlled by way of adjusting a proper control variable, it is possible that in an optimally designed system the effect of the disturbance on the process variable being controlled may become completely eliminated.
Furthermore, the measurement results of web qualities recorded prior to the passage of the web through the calender may also be utilized in feedback control by way of adjusting the modifiable properties of the web to be calendered, such as its moisture content and temperature. In the case that the line includes auxiliary equipment for prewetting and/or preheating the web prior to its entry into the calender, the calender control algorithm can be enhanced through pretreating the web so that those properties of the web to be calendered that affect its processability are modified more compatible with the calendering process.
The invention offers significant benefits.
In both of the above-described embodiments, the calender control algorithm can be improved substantially in regard to a situation, wherein the state of the web being calendered is measured only downstream after the calender. Due to the improved control scheme, the runnability of the calender is improved and the broke produced on the calender is reduced. Measurement results obtained upstream from the calender may also be utilized in the fault diagnostics of the calender. On the basis of the measurement results, it is easy to make a quick diagnosis as to the origin of a given disturbance, whether caused by the calender or other subprocesses of the papermaking machine, whereupon the situation can be corrected toward a better calendering result in a manner superior to the prior art. Obviously, the measurement of qualities of the web to be calendered both upstream from the calender and downstream therefrom also makes fault identification easier in web profile problems.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.